The spreading resistance technique is known for profiling a variety of silicon structures. In this technique, a cross section of the structure (usually a small segment of a wafer) is exposed by beveling at an angle to the wafer surface. With a pair of point contacts, the spreading resistance is measured at regular intervals along the exposed surface to obtain the spreading resistance profile. Experimental details have been set forth in standardized test methods: ASTM F672-88, Standard Test Method for Measuring Resistivity Profiles Perpendicular to the Surface of a Silicon Wafer Using a Spreading Resistance Probe; ASTM F723-88, Standard Practice for Conversion Between Resistivity and Dopant Density for Boron-Doped and Phosphorous-Doped Silicon; and ASTM F674-92, Standard Practice for Preparing Silicon for Spreading Resistance Measurements.
Currently, spreading resistance profile (SRP) measurements are made by first lapping or grinding a sample of semiconductor at a known small angle; mounting the sample on the measuring apparatus so that the original surface and lap surface are in known positions; aligning the sample with manual controls while looking through a microscope so that the position of the start of the beveled (lapped) surface is in a defined starting position and so that the edge of the bevel is perpendicular to the measuring direction; manually moving the sample to a fixed distance from the viewing position to a measuring position adjacent a pair of point contacts (probes); making a series of electrical resistance measurements in fixed increments (typically 1 to 5 microns apart) down the bevel; and analyzing the data of resistance versus distance along the bevel to measure resistivity, dopant density, carrier concentration or other parameters of interest as a function of depth in a semiconductor material.
Prior to making the measurements, the shape of the probe tip is prepared by setting the tip on a surface coated with diamond particles and manually moving the surface to grind away material on the probe tip and to shape the tip.
Finally, prior to making the measurements, the system is calibrated by measuring the resistivity of a sample of bulk material which has known resistivity throughout the sample and has a recently ground surface (to remove oxides) and repeating the measurement on 10 or more individual samples covering the range of interest.
Apparatus have been developed to carry out the spreading resistance technique with some degree of automation, stepping the stage and specimen beneath the point contacts (probes), raising and lowering the probes and logging data. See, for example, U.S. Pat. No. 3,628,137 issued Dec. 14, 1971 to Robert G. Mazur entitled "Apparatus for Automatically Determining Spreading Resistance, Resistivity and Impurity Concentration in Semiconductor Bodies". This patent, incorporated herein by reference, discloses an incremental advance of the specimen while the probes have been automatically raised away from the specimen.
Even with the automation of the stepping of the specimen stage and data logging in the conduct of spreading resistance measuring, the technique is tedious and subject to variable results. In other words, different laboratory technicians will not necessarily develop the same profiles for the same specimen due to experimental variations.
It is an object, according to this invention, to further automate the various specimen handling and aligning, calibration and probe tip configuration steps to eliminate the need for constant technician attendance and to increase the reproducibility of these steps. It is an advantage, according to this invention, that all the motions of the measuring apparatus including lifting and lowering probes and x-y-z-.theta. positioning of the stage can be controlled by a computer. It is a further advantage that the computer will store certain process recipes for preparing the probes, making measurements, and so forth in a highly repeatable manner. It is a still further advantage of this invention that the image of the samples acquired by video camera can be captured in the computer and displayed when necessary. It is a yet further advantage of this invention that the calibration fixture will mount multiple samples and may allow for surfaces of all of the samples to be prepared simultaneously without removing them from the fixture. It is a still further advantage of this invention that a probe conditioning fixture will have an angled shaping surface which may be rotated completely around the axis of the probe tip, multiple surfaces with different diameters of diamond, pads to clean the tip between conditioning steps, and probe qualification samples to check the performance of the probe.